BEIJING, China (CNN) -- Chinese officials in the northwestern Xinjiang Uygur Autonomous Region reported the death of a fourth baby Thursday in the country's expanding contaminated infant formula case. The tainted milk powder has already sickened more than 6,200 babies.

Earlier in the day, authorities announced the arrest of 12 more suspects in the investigation, provincial police said.

The arrests bring to 18 the number of people detained, according to the Hebei Public Security Bureau.

Twelve of those arrested are involved in the collection of milk and the production of milk products, authorities said. Authorities accuse the other six of illegally selling the chemical added to the milk.

More than 1,300 infants are hospitalized, said Li Changjiang, China's director of quarantine and inspection, on Wednesday. Their conditions include malnutrition, kidney stones and acute renal failure.

Inspectors have been deployed across the country to test producers' inventories.

Two brothers who sold fresh milk used to produce contaminated baby milk powder were arrested by Chinese investigators Monday. They could face death if convicted, according to China Daily, a state-run newspaper.

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The raw milk had been watered down and a chemical added to fool quality checks, the newspaper said.

Investigators said the brothers confessed to watering down the raw milk and mixing in tripolycyanamide, also known as melamine. They said they did it to recover losses suffered when the factory rejected earlier milk shipments, the paper reported. Learn more about the chemical melamine »

The brothers are charged with producing and selling toxic and hazardous food, which carries a possible death penalty, the paper said.

Thousands of tons of the tainted milk powder have been recalled, including pullbacks by Mengniu Dairy Group, China's largest milk producer, and the Sanlu Group.

Chinese investigators have found melamine in nearly 70 milk products from more than 20 companies, Li said Wednesday. Products made by Sanlu had the highest concentration of the chemical.

It is not the first time Sanlu has been connected to a scandal involving tainted milk powder, according to China Daily.

In 2004, at least 13 infants in the eastern Anhui province died of malnutrition after drinking milk powder that had little to no nutrition. The illegally manufactured milk was falsely labeled with the Sanlu brand, according to the paper.

The vast majority of the tainted formula was consumed domestically, but it is known to have been exported to Taiwan, Bangladesh, Myanmar, Yemen, Chad and Burundi, according to Li.

The U.S. Food and Drug Administration said no Chinese baby formula has been allowed on the market in the United States. In a statement on its Web site, the FDA said it had reached out to all five companies making formula in the United States and none has used formula or source materials from China.

This episode marks the latest in a string of tainted products produced in China:

Thousands of pets in the United States became ill, some dying, last year after eating food imported from China tainted with melamine, the same chemical found in the powdered milk.

Last October, at least 69,000 Chinese-made toys were recalled in the United States over concerns of excessive amounts of lead paint

In November, it was found that the popular toy Aqua Dots was contaminated with a toxic chemical that turned into a powerful "date rape" drug if swallowed

In February, a Maryland candy distributor pulled Pokemon-brand Valentine lollipops from store shelves after bits of metal were found in the sealed treats, authorities said.